Opiate addicts typically consume a variety of psychoactive drugs, including caffeine. Caffeine antagonizes adenosine receptors and could block some of the effects of opiates. It also aggravates opiate withdrawal symptom after administration of narcotic antagonists in animals and people. This study will test the hypothesis that higher levels of caffeine consumption are associated with greater daily opiate withdrawal symptoms and/or higher methadone doses. The GCRC will be used for data collection (interview) and obtaining blood samples.